In perinatal rat fetuses, the anterior subcutaneous depot contains exclusively brown adipocytes and is recognisable from day 15 of gestation. At the site where the anlage arises, few dilated capillaries are immersed in an extracellular matrix poor in cells and collagen fibrils and rich in amorphous substance. At this stage pericytes - which cannot be defined as 'pure' undifferentiated cells because of their characteristic mitochondria - are already recognisable. They are often numerous, large and rich in cristae (pretypical mitochondria), anticipating their differentiation into typical brown mitochondria. Another distinctive feature is the variable amount of glycogen (Fig. 6). Anlage cellu-larity, but not cell differentiation, increases on the following days.
Around day 18 of gestation, cell number and degree of differentiation augment greatly through the arising of further differentiation features of mitochondria, which become typical; the first lipid vacuoles also appear. Of note, lipid accumulation seems to be multilocular from the very first steps of differentiation.
Around day 20, mitochondria express the functional protein, and all the morphological features of mature brown adipocytes are detected at this time in most cells in the depot. In the first few postnatal weeks, elements at various stages of differentiation continue to be seen.
All such features of brown adipocytes are recognisable in subcutaneous BAT of human fetuses (our unpublished data).
Therefore, also in the case of BAT, stem cells cannot be identified based solely on morphological features; in fact, the most undifferentiated cells found in the pericytic position already exhibit minimal features of differentiation that allow them to be distinguished from 'pure' stem cells.
There thus seem to be two different types of precursor. Both appear to be programmed to form the organ's parenchyma independently of the animal's functional requirements; indeed, the complete absence of |31, 2 and 3 adrenergic receptors (which mediate thermogenic activity) does not prevent the arising of brown preadipocytes with a normal ultrastructure (our unpublished data), and the nearly complete absence of insulin receptor (which mediates liposynthesis) does not prevent the formation of either type of precursor in the adipose organ, even though they do not attain complete differentiation into mature adipocytes .
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